In numerous applications, it is necessary to rapidly cool molten, flowable or like products which may be less than coherent, in order to provide a coherent sheet, film or foil, the materials to be transformed into the layer being at a relatively high temperature. This is especially the case in the plastic industry and in the rubber industry.
In certain cases, especially when the products can have a crystalline nature depending upon the speed of cooling, the quality of the product obtained in the form of the film, foil or sheet will depend largely upon the cooling speed. For this purpose, the use of a cooling drum having a cooled cylindrical surface has been developed in the art.
Since the efficiency of a cooling drum depends, to a large part, on the intimacy of contact between the inner surface of the film and the external surface of the drum, it is important to hold those two surfaces in intimate contact over the entire width of the layer.
More generally, when the foil or sheet, extruded from an extrusion head or the like arrives at the drum without any precautions to insure intimate contact, the foil has a tendency not to adhere to the surface of the drum and, because of irregular cooling, deforms and can be completely unusable.
To remedy this drawback, a large number of prior art devices referred to as emplacement devices or placing devices have been used. One of these devices, which is widely used at the present time, presses the product to be cooled against the surface of the drum by a jet of air at high velocity at high pressure to insure an intimate contact of the foil with the drum; and to smooth the foil uniformly against the drum. Such devices are generally referred to as sheets of air.
An air sheet device for applying a foil or film against the drum can be constituted by a tubular body disposed transversely of the path of the foil above the drum, fed by a blower or a compressor and opening between two longitudinal lips formed at the end of a convergent portion into a slit which discharges a sheet of air oriented substantially radially with respect to the drum.
The level of elevated pressure which is maintained in the interior of the tubular body, thanks to the angle of convergence of the lips, insures an elevated speed of the flow of air which is effective to apply the foil or film to the drum.
The uniformity of the emplacement of the foil or film over the width thereof depends upon the uniformity of the opening between the lips and for different systems utilizing differential screw arrangements or even more sophisticated systems with deformable flexible lips, the gap between the lips can be controlled in order to obtain a jet which is homogeneous over the entire width of the film.
It should be evident that for uniform emplacement of the film, the width of the jet should be at least equal to the width of the film. If the width of the jet is less, emplacement or pressing force is insufficient along the edges of the film.
This problem of sufficient pressure at the edge is a primary source of problems for the prior art devices of the type described.
Depending upon the material from which the foil or film is constituted and the speed with which the foil material is deposited upon the drum, the width of the foil can vary on the drum to a greater or lesser extent.
For this reason, it is the current practice to utilize a width of the blowing device which exceeds the width of the film and under all circumstances under which the film or foil is deposited on the drum. Indeed, I am not aware of any system in this context which can adjust the width of the air jet to the width of the film without generating along the edges of the film turbulence which can cause the edges to lift away from the drum and effect the emplacement.
The provision of an airflow across the width of the film or foil which can extend beyond the edges, moreover, leads to a nonuniform effect as well as a result of the fact that upon extrusion, the edges of the foil or film can have a thickness which is generally greater than that of the rest of the film to minimize the tendency of the film to curl toward the face of the drum. A high speed jet at the edge can allow the passage of air underneath the foil where there is a tendency to curl and thus can result in a separation of the foil from the drum and nonuniform cooling. Particularly in the case of bi-oriented films it is common to use between the extrusion head and the point of impact of the air, supply air jets directed against the edges and whose function is to emplace the edges of the film upon the drum and thereby create a flattening effect before the film reaches the point of impact of the main air curtain utilized to emplace the film against the drum. While this system is effective to obviate some of the drawbacks enumerated above, it is necessary to mount the supplemental jets on high accuracy positioning devices and to set the angular orientation of the jet with precision for a particular film. Regrettably, the edges of the foil or film are of an unstable nature and change as a function of a temperature of the material or the viscosity thereof readily. The edge jets, therefore, are difficult to control and constitute a perpetual force of trouble.